Secondary or rechargeable battery packs are in widespread use as the principal power source for mobile equipment such as cellular telephones and portable computers. As such, it is desirable to have a lightweight battery pack that stores an adequate amount energy so that the user of the battery powered device is not inconvenienced with having to carry spare battery packs, or stop using the device while the battery pack is recharged. Recently a number of lithium based battery chemistries, such as the so called lithium ion battery has appeared, and provides a superior energy storage to weight ratio over conventional chemistries such as nickel/cadmium and nickel/metal-hydride.
However, lithium batteries made for consumer electronics require safety circuitry within the battery pack to prevent excessive voltage levels from being applied to the battery cell or cells. Should an excessive voltage be applied to such a cell, the cell could malfunction and pose a safety hazard. Safety circuits must therefore be robust and able to withstand reasonable electrical stress, such as that caused by electrostatic discharge events.
An electrostatic discharge event occurs when a person has become charged with static electricity, and then dissipates the charge into the battery pack upon coming into intimate proximity with the battery pack. Many studies have been done to characterize static charge accumulation on humans, and it is not uncommon for voltage levels to reach as high as 20 Kilovolts (kV) under favorable conditions. However, it is more likely that a typical person, under normal circumstances, will achieve a peak static voltage level of 10 kV to 15 kV. Even at such high levels, conventional circuit design techniques can be used to prevent damage of the safety circuit from occurring at these levels, but it is a much more difficult task to prevent the mis-operation of the safety circuit upon a discharge event at these level, particularly when the battery voltage is near an upper voltage limit.
A mis-operation of a battery circuit can result in the battery pack being temporarily disabled until the safety circuitry can be reset. Obviously a disabled battery pack can be problematic for a user, especially if the user does not have the means to reset the safety circuit at hand. Therefore there exists a need for an electrostatic enhanced lithium battery circuit that can withstand high level electrostatic discharge (ESD) e vents without a mis-operation.